Firefox users who are running the Windows Presentation Foundation (WPF) plug-in and do not have it disabled should also apply this security update.

If I use Firefox, which Internet Explorer update do I need to install? If a computer system is configured for Automatic Update, the correct update will be downloaded and made available for installation depending on the Automatic Update configuration. In the event that a computer system is not configured for Automatic Update, users should verify which version of the Windows operating system and Internet Explorer is on their system and download the appropriate update.

If I install this security update, do I need to disable the Windows Presentation Foundation Plug-in in Firefox to be protected from this vulnerability? No. Customers who have installed the security updates associated with this security bulletin are protected from this vulnerability.

If I have not yet applied this security update, how do I disable the Windows Presentation Foundation plug-in in Firefox?If you have not yet applied this update, you can disable the Windows Presentation Foundation plug-in in Firefox to block this vulnerability. To do this, launch the Firefox browser, select the Tools pull-down menu, and then click Add-ons. Select the Plugins icon at the top of the Add-ons window. In the list of Plugins, select Windows Presentation Foundation 3.5.30729.1 and click Disable.

If I uninstall the .NET Framework Assistant extension, does it disable or remove the Windows Presentation Foundation plug-in?If the .NET Framework Assistant extension is uninstalled it does not disable or remove the Windows Presentation Foundation plug-in. The .NET Framework Assistant and Windows Presentation Foundation plug-in are controlled through different screens in the Firefox Add-ons management window.

The initial version of the advisory didn't mention Firefox at all which created a problem for Mozilla. Concerned that Firefox users wouldn't feel the need to apply the update, even though it was labeled "Critical," Mozilla decided to block the use of two Microsoft add-ons through their blocklisting mechanism. The fact that these add-ons installed on users' systems without notice during installs of related Microsoft software probably figured into this decision. On the other hand, such quiet installed of Firefox add-ons are not uncommon, but Mozilla disabling them is.
The status of the issue this morning indicates that it was, at heart, a miscommunication.